Compositions comprising a vinylaromatic polymer and a rubber must exhibit a set of properties such as good impact resistance, a gloss suited to the desired application and a melt index suited to the conversion technique envisaged.
It is useful to be able to have available means which make it possible to influence these various properties so as to control them better and to make them more suitable for the targeted application.
It is known that compositions comprising a rubber in the form of nodules are generally more resistant to impacts when the distribution of the sizes of the rubber nodules is broad, indeed bimodal. Various processes have been proposed for producing this type of material. Patent EP 48389 teaches that a high-impact polystyrene with the bimodal distribution of nodules can be obtained by independent manufacture of two compositions each containing a specific population of nodules and then by mixing these two compositions. Patent Application EP 418042 teaches that such a composition can be obtained using a single polymerization reactor, by virtue of the use of a polybutadiene exhibiting a bimodal molecular mass distribution.
Patent Application WO 94/11412 teaches that polystyrene with a polydispersity of less than 2 and with a weight-average molecular mass of less than 80,000 can be obtained when the polymerization of styrene is carried out in the presence of a stable free radical and of a polymerization initiator and in the absence of rubber, the molar ratio of the stable free radical to the polymerization initiator being between 0.4 and 2.5, which corresponds to a concentration of stable free radical initiator which is markedly greater than 0.1% by weight in the polymerization mixture and more generally in the region of 1% by weight in the polymerization mixture.